The ability of Legionella pneumophila (Lpn) to cause pneumonia is dependent on intracellular replication within pulmonary phagocytic and epithelial cells. In the environment, the bacteria are ubiquitous where they multiply intracellularly with amoebae. Invasion and intracellular replication within protozoa play major factors in the amplification and dissemination of Lpn in the environment and in transmission and infectivity to humans. We have recently shown that uptake of Lpn by the protozoan Hartmannella vermiformis is mediated by bacterial attachment to a b2 integrin-like Galactose/N-acetyl-galactosamine lectin on the protozoan surface. Bacterial attachment to the lectin is associated with induction of protozoan gene expression and with tyrosine dephosphorylation of the lectin and several cytoskeletal proteins including actin, focal adhesion kinase, paxillin, and vinculin. Our data suggested a cytoskeletal disruption in the protozoan host upon bacterial attachment. Transmission electron microscopy showed that these bacterial-induced manipulations of cell processes in the protozoan host are associated with entry of the majority of the bacteria through a cup shape-like invagination that resemble receptor-mediated endocytosis, but some bacteria are internalized by coiling phagocytosis. Our preliminary data suggest that the mechanism of entry of Lpn is novel, which may contribute to its subsequent intracellular fate. Our hypothesis is that the lectin is a protozoan receptor involved in uptake of Lpn, and is dissociated from the cytoskeleton upon bacterial attachment and invasion. Our specific aims are, 1) to clone the lectin encoding gene and examine its regulation of expression; 2) to clone and characterize the bacterial ligand that binds the lectin receptor and the mode of ligand-receptor interaction; 3) to determine cellular distribution of the lectin receptor and its subsequent fate after internalization; and 4) to evaluate the interaction of the receptor with the cytoskeleton. The results derived for the proposed studies will uncover new paradigms of uptake of intracellular pathogens and will contribute to our understanding of targeting of molecules into a "protected vacuole" inside eukaryotic cells. Our proposed studies may also facilitate the design of future preventive strategies to control the amplification and spread of Lpn in the aquatic environment, which is the only source of bacterial transmission to humans. Our studies may uncover potential pathogenic evolution of Lpn to invade the more evolved mammalian cells, and may contribute to the understanding of invasion of protozoa by Mycobacterium and Chlamydia.